The present invention relates generally to a distributed feedback semiconductor laser (hereafter referred to also as xe2x80x9cDFB laserxe2x80x9d) in which optical feedback or light feedback is performed by using a diffraction grating, and more particularly to a DFB laser in which return light induced noises or external feedback noises can be decreased to reduce fluctuation of an optical output thereof.
Conventionally, a semiconductor laser is used as a light source for optical communication. A part of a laser light emitted from the light source is reflected by one or more optical components such as an optical connector and the like disposed on an optical path. When a reflected return light (or reflection return light, or external optical feedback), that is, a light reflected by the optical components and returning backward, is incident on the semiconductor laser as the light source, return light induced noises or external optical feedback induced noises are produced within the semiconductor laser. That is, optical output level of the semiconductor laser fluctuates. When the optical output fluctuates, there arises a possibility of transmission code error.
As a method of preventing the reflected return light from entering the semiconductor laser, it is considered possible to provide an optical isolator on the side of the emission end or the outlet end of the semiconductor laser. However, when the optical isolator is used, the optical isolator is itself expensive, and manufacturing process of the light source also becomes complicated, so that manufacturing cost of the light source becomes high.
Therefore, a DFB laser is proposed in which generation of the return light induced noises can be suppressed without using the optical isolator. One example of such DFB laser is disclosed in a document 1, i.e., Japanese patent laid-open publication No. 4-17384 (Japanese patent application No. 2-120026). According to a technique disclosed in this document 1, a DFB laser, in which optical feedback or light feedback is performed by using a diffraction grating, is divided into two regions along the length of a resonator thereof. Also, one of the regions on the side of the emission end is used as a non-excitation region, and the other region is used as an excitation region, that is, a current injection region. Therefore, an electrode for injecting current are provided only on the upper surface of the excitation region. By using such structure, it is possible to utilize a diffraction grating of the non-excitation region as a distributed reflector. As a result, it is possible to prevent the reflected return light from coming into an active layer of the DFB laser.
However, in the technique disclosed in the above-mentioned document 1, reflectance, of the distributed reflector in the non-excitation region, for the output emission light of the DFB laser is the same as reflectance for the reflected return light. As a result, when the reflectance of the distributed reflector is made high, optical loss in the non-excitation region also becomes large, and an oscillation threshold of the DFB laser becomes high. Therefore, it becomes difficult to sufficiently suppress incidence of the reflected return light into an active layer.
It is therefore an object of the present invention to obviate the above-mentioned disadvantages of the conventional distributed feedback semiconductor laser (DFB laser).
It is another object of the present invention to provide a DFB laser which has a high immunity against reflected return light.
It is still another object of the present invention to provide a DFB laser in which fluctuation of an optical output of the DFB laser caused by the reflected return light can be suppressed.
In order to attain the above-mentioned objects of the present invention, the inventor of the present invention found, after performing various experimentation and consideration, a phenomenon that quantity of detuning of a semiconductor laser exhibits a fluctuation of opposite phase to a fluctuation of an optical output, when the optical output of the semiconductor laser fluctuates.
In this case, the quantity of detuning xcex4xcex2 is designated by the following formula.
xcex4xcex2=2neqxcfx80((1/xcex)xe2x88x92(1/xcexB))xe2x80x83xe2x80x83(1)
where, neq designates an equivalent refractive index of an active layer of a semiconductor laser, xcex designates an oscillation wavelength of the semiconductor laser, and xcexB designates Bragg wavelength.
Therefore, the inventor thought out that, by suppressing the fluctuation of the optical output caused by the reflected return light by utilizing the phenomenon that quantity of detuning of a semiconductor laser exhibits a fluctuation of opposite phase to a fluctuation of an optical output, it is possible to improve the immunity of a semiconductor laser against the reflected return light. Also, the inventor derived the condition that optical output fluctuation caused by the fluctuation of the quantity of detuning of a distributed feedback semiconductor laser becomes a negative feedback with respect to optical output fluctuation caused by the reflected return light, and thereby thought out a technical idea of the present invention described below.
According to an aspect of the present invention, there is provided a distributed feedback semiconductor laser comprising: a diffraction grating structure portion which constitutes a resonator and which is divided into a plurality of regions along the longitudinal direction of the resonator; and one or more phase shift portions each disposed between adjacent regions of the diffraction grating structure portion; wherein total phase shift obtained by all of the phase shift portions has a quantity corresponding to xcex/n, where xcex is an oscillation wavelength, and n is an integer larger than 4 (n greater than 4).
As mentioned above, when the optical output of a semiconductor laser fluctuates, the quantity of detuning fluctuates in the opposite phase to that of the fluctuation of the optical output. When the quantity of detuning fluctuates, quantity of reflecting mirror loss which determines an oscillation mode also fluctuates according to the fluctuation of quantity of detuning. When the quantity of reflecting mirror loss fluctuates, an intensity of light emission of a semiconductor laser also fluctuates.
When the quantity of the reflecting mirror loss increases, the optical output of the semiconductor laser decreases. On the other hand, when the reflecting mirror loss decreases, the optical output of the semiconductor laser increases. Therefore, depending on the direction of the fluctuation of the quantity of reflecting mirror loss due to the fluctuation of the quantity of detuning, fluctuation of the optical output caused by the reflected return light is amplified or suppressed.
It is known that the direction of the fluctuation of the reflecting mirror loss with respect to the fluctuation of the quantity of detuning depends on the quantity of phase shift provided at the diffraction grating structure portion of a DFB laser. That is, when the phase shift is larger than a quantity corresponding to xcex/4, where xcex designates an oscillation wavelength, the reflecting mirror loss decreases (increases), in accordance with the decrease (increase) of the quantity of the detuning, respectively. Therefore, when the optical output of the semiconductor laser increases due to the reflected return light, the quantity of detuning decreases and the reflection mirror loss also decreases. As a result, the optical output of the semiconductor laser further increases. That is, the fluctuation of the optical output due to the reflected return light is amplified. Thus, when the phase shift is larger than xcex/4, positive feedback phenomenon occurs.
On the other hand, when the phase shift is smaller than the quantity corresponding to xcex/4, it is known that the reflecting mirror loss increases (decreases), according to the decrease (increase) of the quantity of detuning, respectively. Therefore, when the optical output of the semiconductor laser has increased due to the reflected return light, the quantity of detuning decreases and the reflection mirror loss increases. As a result, the optical output of the semiconductor laser fluctuates toward decrease. That is, increase in the optical output due to the reflected return light is suppressed. Thus, when the phase shift is smaller than xcex/4, negative feedback phenomenon occurs.
Therefore, by making the phase shift smaller than the quantity corresponding to xcex/4, it is possible to suppress the fluctuation of the optical output of the DFB laser caused by the reflected return light. That is, it is possible to improve immunity against the reflected return light of the DFB laser.
It is preferable that the total phase shift has a quantity corresponding to a value within a range between xcex/4 and xcex/16 and more preferably between xcex/5 and xcex/8.
In the negative feedback region in which the phase shift is smaller than xcex/4, the smaller the phase shift, the larger the fluctuation of the reflecting mirror loss with respect to the fluctuation of the quantity of detuning. Therefore, when the total phase shift is made equal to or smaller than xcex/5, it is possible to secure enough magnitude of the fluctuation of the reflecting mirror loss with respect to the fluctuation of the quantity of detuning. As a result, it is possible to sufficiently suppress the fluctuation of the optical output caused by the reflected return light.
Also, if the phase shift is made equal to or larger than xcex/8, it is possible to keep the fluctuation of the reflecting mirror loss equal to or smaller than a predetermined quantity, when the quantity of detuning fluctuates. As a result, it becomes possible to prevent the quantity of fluctuation of the optical output due to the negative feedback from largely exceeding the quantity of fluctuation of the optical output caused by the reflected return light.
Therefore, if the total phase shift is made to have a quantity corresponding to a value within a range from xcex/8 to xcex/5, that is, a range between a value equal to or larger than xcex/8 and a value equal to or smaller than xcex/5, the fluctuation of the optical output caused by the reflected return light can be effectively suppressed.
Also, it is preferable that the diffraction grating structure portion is divided into a first region and a second region and that a phase shift portion is provided between the first and second regions.
By using a structure in which two regions of diffraction grating structure are provided, that is, by using a structure in which one portion of the phase shift portion is provided therebetween, the structure of the DFB laser can be simplified.
It is also preferable that the phase shift portion is disposed at the central portion along the longitudinal direction of the resonator, that an average period of all of said regions of the diffraction grating structure portion is determined as a reference period, that a period of the first region of the diffraction grating structure portion is increased with respect to the reference period, and a period of the second region of the diffraction grating structure portion is decreased with respect to the reference period, and that an increment of the period of the diffraction grating structure portion in the first region and a decrement of said period of said diffraction grating structure portion in the second region at any equal distances from the phase shift portion are equal to each other.
If the diffraction grating of the DFB laser has the above-mentioned structure, it is possible to mitigate concentration of electric field in the phase shift portion, and to make an internal electric field of the DFB laser uniform. As a result, it becomes possible to make carrier distribution uniform and, therefore, to suppress occurrence of spatial hole burning.
It is preferable that the phase shift portion is disposed at a portion shifted toward the front end surface from the central portion along the longitudinal direction of the resonator.
By providing the phase shift portion at a location near the front end surface, it is possible to raise an electric field strength near the front end surface which is an emission end surface of the DFB laser. Therefore, an output efficiency of the DFB laser can be improved.
It is also possible to divide the diffraction grating structure portion is into at least three regions and to provide a phase shift portion each between adjacent regions.
By providing a plurality of phase shift portions, it is possible to mitigate concentration of electric field in the phase shift portions, and to make an internal electric field of the DFB laser uniform. As a result, it becomes possible to make carrier distribution uniform and, therefore, to suppress occurrence of spatial hole burning.
It is preferable that the phase shift portion has a phase shifting diffraction grating structure which has a period different from that of the diffraction grating structure portion in the plurality of regions.
By making a phase shift portion have diffraction grating structure having different period in this way, it is possible to make the width of the phase shift portion along the length of the resonator large. To this end, it is possible to mitigate concentration of electric field in the phase shift portion, and to make an internal electric field of the DFB laser uniform. As a result, it becomes possible to make carrier distribution uniform and, therefore, to suppress occurrence of spatial hole burning.
Also, it is preferable that the diffraction grating structure portion is formed at an interface portion between an optical guide layer and another layer adjacent the optical guide layer.
Further, it is preferable that the diffraction grating structure portion has a diffraction grating structure of gain coupling type in which optical gain distribution of an active layer varies periodically along the longitudinal direction of the resonator.